I've updated my rear brakes. Install is finished and I have bled the brakes, all four corners just to be sure...started with right rear, then left rear, then right front and last the left front. The pedal is great, very firm. My problem is when I start the car the brake pedal becomes spongy and slowly goes to floor. When I shut car off the pedal becomes firm again. Any suggestions or answers on what is going on?

One of your lines is probably not tight enough, my car did the exact same thin after I swapped in new rear lines calipers and rotors. My lines were just leaking a very small amount, tightened them up a bit more and the problem was fixed.

It is normal to have some difference in pedal feel between engine on and engine off because there is a booster that helps with braking when the engine is running. Perhaps your brake fluid is too low, or it could be that air got into the lines if they weren't bled properly. Did you just open the lines to let them drain some or did you have someone pressing the brake pedal? And did the M/C fluid ever go dry?

The brakes were bled two different ways. First I bled them with a power bleeder. Second time was with a helper pumping the brake pedal. Both times I bled them starting from the Rt (passenger) rear, then the Left (Driver's side) rear, then the Rt Front and lastly the Left front. With a power bleeder you are always feeding fluid into the m/c. When using a helper I checked the m/c after every bleed, so the m/c never went dry.

I think I'm gonna try bleeding them while the car is running as that is when the pedal gets spongy. I think that there is something going on with the ABS myself. There are solenoids that turn on and off in the ABS and I'm thinking that maybe they are switching on or off (?) when the car is started and allowing air to enter lines. After that if that doesn't work I guess it's off to the dealer and question them.

And what you are not understanding is the fact that when the car is not running I have brakes and the brake pedal is where it is suppose to be. But, when I start the car the brake pressure goes away. And when I turn it off the brake pedal and pressure comes back to where it is suppose to be.

If your ABS system was cycling on and off your pedal would pulsate not be spongy. Do not bleed your brakes with the car running that is not the right way to do it. A spongy pedal is indicative of air in the system. If I were you I would gravity bleed the system crack one bleeder screw at a time and let bleed for at least 10-20 min then close it and go to another wheel and do the same untill you have done all of them. Another thing is if your pedal creeps to the floor when the car is running with light pressure on it your primary cup seal on the primary piston could be cut causing an internal leak in the master cylinder.

But then explain to me why, when I've bled all four wheels, with a pressure bleeder (Motive) and once with a helper pumping and holding the brake pedal till the brake pedal is very firm and exactly where, in my opinion, it should be that it gets spongy and goes to the floor only when the car is running?

Well, as of yesterday, I went to a shop that a friend owns and he recommended that I get the complete system power bled. I guess there is a difference in power bleeding and pressure bleeding. So, I'm having the car picked up with a flat bed tow truck and taking it to him. He's not charging me for it (that's what good friends are for..lol) and I will get it back in a couple of days. I will post exactly what they had to do to resolve the problem.

Well, here's an update on my bleeding situation.....I have aftermarket axles (Strange) in my rear end. The axle hub is .022 thicker than stock. This causes the rotors to be off center of the calipers. So they are always in contact. That in it self is not right. The mechanic working on it has done these systems before and at this point doesn't seem to think there is any air in the system. But once he has this snafu taken care of (going to shim caliper out) he will then double check for air in system again and may check or bleed the control system (valves) and bleed if necessary. Will be done in the morning.

Last update on brake bleeding...wo0t!! Car is done, brakes are fabulous. Now, the problem.....The reason I had a hard pedal when not running and a spongy pedal when running was due to two factors. First...the caliper wasn't centered over the rotor because I have strange axles installed. The caliper hub on the axle is .022 thicker than stock. We had to shim the caliper out to solve that. Secondly the solenoids had to be cycled as the brakes were being bled and they did that with the car running. Certainly glad that snafu is over and I got to drive my Mustang today and that in itself was fantastic...!!

I have never heard or read that the solenoid have to be cycled why the brakes are being bled. I just re did my rotors and pad but removed the calipers to paint them. How do you cycle the solenoids then?

If you have a shop manual it will tell you about it in there. They use a scanner that hooks up to your OB II port under your dash and a laptop computer. I watched the tech do it, and it's quite interesting. It's not something that has to be done every time you replace pads. But if you get air up into the system then that is when you have to do it. Don't ask how I got air that far up into the system as I didn't let the m/c cylinder fluid level get to low and I bled the system correctly but that's what part of my problem was. I have the complete shop manuals printed out for Ford shops, but, I would think even a Haynes would mention it.

In ABS systems to do a proper bleed you should cycle your ABS solenoids if you have a scan tool with bi directional controls you can do it that way if not you can just turn it on and it should cycle but some systems won't cycle untill you reach a speed between 5-10 mph so that makes it kind of difficult to do it